In the manufacture of integrated circuits, higher resolution is required for the exposure transfer of circuit patterns. Since resolution is known to be inversely proportional to exposure wavelengths the exposure wavelengths have been designed to have shorter waves. In recent years, KrF excimer laser (248 nm) and ArF excimer laser (193 nm) have drawn attention for use as the exposure light source. Due to the limitation in transmittance, the composition of the glass materials, used in the oscillated wavelength ranges, for the above ultra violet lasers are limited. Also, the dispersion of refraction index of a glass material is larger in the oscillated wavelength range for the ArF laser compared to the KrF laser. As such a decrease in imaging performance due to chromatic aberrations will become noticeable. In order to solve the above problem, several methods have been proposed with one method involving narrowing the band of the spectrum in the laser such that it is almost a monochromatic light source and with another method involving compensating for chromatic aberrations in the projection optical system.
However, in order to provide complete compensation for chromatic aberrations over a wide spectrum range for a given source of laser illumination, it is essential to combine a fluorite lens having strong positive power and a quartz lens having strong negative power. Since the glass materials mentioned above have extremely small difference between each other in dispersion of index of refraction this limits complete compensation for chromatic aberrations and makes compensation of other monochromatic aberrations extremely difficult which, in turn, prevents the projection lenses from having improved imaging performance. Also, it makes tolerance of decentering strict and assembly difficult.
The present invention provides a projection optical system in which both problems of partial compensation of chromatic aberrations and compensation of other monochromatic aberrations may be handled simultaneously. Therefore, the range of spectrum of a laser light source for exposure can be set wider so that the throughput is improved.